A known seat reclining apparatus is disclosed in JP2002-101996A. The seat reclining apparatus disclosed includes a lower guide 51, an upper gear 52, a ring holder 53, three pawls 54, a cam 55, and a locking spring 56 as shown in FIG. 11. The lower guide 51 is fixed to a seat cushion frame (not shown) while the upper gear 52 is fixed to a seatback frame (not shown). An outer peripheral face 52a of the upper gear 52 and an inner peripheral face 51a of the lower guide 51 are slidably engageable with each other so that the lower guide 51 and the upper gear 52 are relatively rotatable to each other. In addition, the lower guide 51 and the upper gear 52 are connected to each other through the ring holder 53 so as not to separate from each other.
The seat reclining apparatus disclosed farther includes a mechanism for retaining an angle of the upper gear 52 relative to the lower guide 51. That is, as shown in FIG. 12, the upper gear 52 includes internal teeth 52b on an inner peripheral side. Meanwhile, the three pawls 54 include external teeth 54a on an outer peripheral side engageable with the internal teeth 52b of the upper gear 52. The cam 55 rotates, while being guided by the lower guide 51, to move the three pawls 54 in a radial direction so as to engage or disengage the external teeth 54a of the three pawls 54 with the internal teeth 52b of the upper gear 52. The cam 55 is operated to rotate in association with the operation of a lever shaft 58 connected to an operation lever (not shown). The locking spring 56 biases the cam 55 in a direction in which the pawls 54 engage with the internal teeth 52b of the upper gear 52. The cam 55 and the pawls 54 are guided by guide portions 57 formed on the lower guide 51. Further, a biasing spring is normally arranged between the seat cushion frame and the seatback frame for biasing the seatback frame in a forward direction though it is not disclosed in JP2002-101996A.
According to the aforementioned seat reclining apparatus, in order to enable a relative rotation between the upper gear 52 and the lower guide 51, a predetermined clearance C is formed between the outer peripheral face 52a of the upper gear 52 and the inner peripheral face 51a of the lower guide 51 as shown in FIG. 12. In case of changing the angle of the seatback relative to the seat cushion, the cam 55 rotates so as to release the engagement of the pawls 54 with the internal teeth 52b of the upper gear 52. The upper gear 52 may be movable and eccentric to the lower guide 51 within the clearance C accordingly. When the engagement of the pawls 54 with the upper gear 52 is released so as to tilt the seatback, the seatback frame normally receives a load in a substantially downward direction due to a weight of the seatback, a load added by an occupant to the seatback, and the like. Thus, the upper gear 52 fixed to the seatback frame is shifted in the downward direction from the lower guide 51 fixed to the seat cushion frame by an amount corresponding to the clearance C. That is, a center axis of the outer peripheral face 52a of the upper gear 52 is made offset in the downward direction to the inner peripheral face 51a of the lower guide 51. A pivot axis of the cam 55 guided by the lower guide 51 is also made offset to the internal teeth 52b of the upper gear 52. As a result, engagement or disengagement timings of the three pawls 54 operated by the cam 55 relative to the internal teeth 52b of the upper gear 52 are not equal, which leads to an uncomfortable operation feeling of the seat reclining apparatus.
Thus, a need exists for a seat reclining apparatus that can reduce an effect of an offset between an upper gear and a lower guide resulting from a clearance formed at a rotatably connecting portion between a seatback frame and a seat cushion frame because of a load applied to a seatback frame when a seatback is tilted. Further, a need exists for a seat reclining apparatus that can achieve a simultaneous engagement or disengagement between engaging members and engaged members, respectively.